1. Field of the Invention
The invention relates to a diode and a semiconductor device having such a diode and, more particularly, to a diode which is suitably used in a semiconductor integrated circuit or the like and to a semiconductor device having such a diode.
2. Related Background Art
Generally, a diode used in an electronic circuit device is formed by a PN junction of a semiconductor and is used as various circuit constructing devices taking advantage of its rectifying characteristic, breakdown voltage characteristic, and the like.
In such a diode, an internal electric field distribution is determined by an impurity profile of the PN junction. The characteristics of the breakdown voltage and the like are determined by the internal electric field distribution. Therefore, in general, by controlling the impurity profile of the PN junction, desired electrical characteristics are obtained.
However, various characteristics are required for a diode. When only the impurity profile of the PN junction is controlled, it is difficult to form a diode having the required characteristics.
For instance, it is possible to form a diode having a low capacity and a high processing speed, due to a high operating frequency, by controlling the impurity profile of the PN junction. It is also possible to form a diode such as an ordinary Zener diode which is used as a device for a constant voltage source circuit by using the breakdown voltage as a reference.
There is a method of reducing a capacity of the diode in order to accomplish a high-speed operating frequency. Practically speaking, a low concentration layer is sandwiched between high concentration layers of PN, thereby substantially widening a depletion layer and realizing a field relaxation. According to the above method, however, although a low capacity can be realized, a withstanding voltage rises, so that such a diode cannot be used as a low constant voltage source.
On the other hand, even in the case where a diode is used in a limited application field such as a diode for a high-speed operating frequency or a diode for a constant voltage, in order to form diodes for a plurality of application fields in the same chip, it is necessary to change the impurity profile of the PN junction in every diode. Therefore, manufacturing steps for such diodes become very complicated and it is difficult to suppress a variation of processing conditions.
As mentioned above, when only the impurity profile of the PN junction is controlled, it is difficult to manufacture a diode having both of the characteristics (diode which has a high operating speed and is used as a constant voltage source).